A recent report from my laboratory has shown that a guanine residue is incorporated into the tRNA of mammalian reticulocytes. It will be recalled that these cells do not synthesized RNA. Chromatography on benzoylated DEAE cellulose and Freon reversed phase chromatography have shown that the guanylation phenomenon does not occur in all species of tRNA but is limited to tRNAHis. Analysis of the guanylated RNA has shown that the guanine is covalently linked to the tRNA and is probably incorporated into an internal position of the polynucleotide chain rather than by terminal addition or branching. The biological role of guanylated tRNA remains obscure as does the mechanism by which the base becomes incorporated into the polynucleotide chain. The role, if any, for guanylated tRNA in protein synthesis will be assessed with a cell-free protein synthesizing system that is dependent upon the addition of tRNA. Recently, we have succeeded in obtaining incorporation of a variety of guanine derivatives into tRNA with cell-free extracts and it will now be possible to purify the enzyme(s) that operate in the guanylation phenomenon and define the reactions involved. The guanylation reaction occurs in reticulocytes but not in mature red cells and guanylation of tRNA in the maturation process in erythroid cells will also be investigated. The availability of a cell-free guanylating system opens up the possibility of studying the reaction in other cell types and assessing the role of guanylated tRNA in controlling protein synthesis and cell maturation. The guanylated tRNA and its precursor, unguanylated tRNA, will be purified for determination of their primary structure. Finally, it will be determined if guanylation of tRNA plays any role in blood dyscrasias such as sickle cell anemia or in cells resulting from aberrant differentiation leading to neoplasia.